Wide Carbon Nanopores as Efficient Sites for the Separation of SF6 from N2.

Takase A, Kanoh H, Ohba T - Sci Rep (2015)

Bottom Line:
The high selectivity of SF6 over N2 was observed only in the low-pressure regime in the interstitial 0.7 nm nanopores; the selectively was significantly decreased at higher pressures.In contrast, the high selectivity was maintained over the entire pressure range in the internal 2.9-nm nanopores.These results showed that the wide carbon nanopores were efficient for the separation of SF6 from the mixed gas.

ABSTRACTSF6 and SF6-N2 mixed gases are used widely as insulators, but such gases have high greenhouse gas potential. The separation of SF6 from SF6-N2 mixed gases is an inevitable result of their use. Single-walled carbon nanohorns (CNHs) were used here for a fundamental study of the separation of SF6 and N2. The diameters of the interstitial and internal nanopores of the CNHs were 0.7 and 2.9 nm, respectively. The high selectivity of SF6 over N2 was observed only in the low-pressure regime in the interstitial 0.7 nm nanopores; the selectively was significantly decreased at higher pressures. In contrast, the high selectivity was maintained over the entire pressure range in the internal 2.9-nm nanopores. These results showed that the wide carbon nanopores were efficient for the separation of SF6 from the mixed gas.

Mentions:
Both the internal and interstitial nanopores of the partially oxidized CNHs were available for the adsorption of molecules, whereas the internal nanopores of the as-grown CNHs were closed to adsorbed molecules; thus, molecules could be adsorbed in the interstitial nanopores, as reported elsewhere2324. The internal nanopores were assessed by measuring the difference between the amounts of N2 adsorbed for the partially oxidized CNHs and the as-grown CNHs. The micropore volumes of the interstitial and internal nanopores of the CNHs were determined from the N2 adsorption isotherms measured at 77 K on the as-grown and partially oxidized CNHs, as shown in Fig. 1a. The nanopore volumes of the as-grown and partially oxidized CNHs were 0.13 and 0.56 mL g−1, respectively, as obtained from the Dubinin-Radushkevich equation25. Thus, the nanopore volumes of the interstitial and internal nanopores of the CNHs were 0.13 and 0.43 mL g−1, respectively. The adsorption isotherms and porosities of the CNHs agreed with results from previous studies202122232426. The nanopore size distributions in Fig. 1b were obtained using the Barrett-Joyner-Halenda theory27. The interstitial and internal nanopore sizes of the CNHs were distributed mainly in the ranges of <1.2 nm and 1–4 nm, respectively. Thus, the interstitial and internal nanopores were named as narrow and wide nanopores, respectively. The X-ray photoelectron spectroscopies in Fig. 1c and transmission electron microscopic images in Fig. 1d indicated that geometrical and chemical structures were rarely changed by the partial oxidation of as-grown CNHs. The O/C ratios were approximately 5% and those CNHs had less surface oxygen groups. Thus, we discussed the change of selectivity by nanopore size. Figure 1e,f shows N2 and SF6 adsorption isotherms measured for the CNHs at 273 K. The N2 adsorption isotherms measured at 273 K were Henry-type, and the adsorbed amounts were small. In contrast, SF6 was adsorbed well in these nanopores even at 1 atm, because of the strong intermolecular interactions of the SF6, corresponding to P/P0 = 0.07. Because the amounts of SF6 adsorbed in the interstitial and internal nanopores were significantly different, the SF6 was less densely packed in the interstitial, narrow nanopores with a diameter of 0.7 nm.

Mentions:
Both the internal and interstitial nanopores of the partially oxidized CNHs were available for the adsorption of molecules, whereas the internal nanopores of the as-grown CNHs were closed to adsorbed molecules; thus, molecules could be adsorbed in the interstitial nanopores, as reported elsewhere2324. The internal nanopores were assessed by measuring the difference between the amounts of N2 adsorbed for the partially oxidized CNHs and the as-grown CNHs. The micropore volumes of the interstitial and internal nanopores of the CNHs were determined from the N2 adsorption isotherms measured at 77 K on the as-grown and partially oxidized CNHs, as shown in Fig. 1a. The nanopore volumes of the as-grown and partially oxidized CNHs were 0.13 and 0.56 mL g−1, respectively, as obtained from the Dubinin-Radushkevich equation25. Thus, the nanopore volumes of the interstitial and internal nanopores of the CNHs were 0.13 and 0.43 mL g−1, respectively. The adsorption isotherms and porosities of the CNHs agreed with results from previous studies202122232426. The nanopore size distributions in Fig. 1b were obtained using the Barrett-Joyner-Halenda theory27. The interstitial and internal nanopore sizes of the CNHs were distributed mainly in the ranges of <1.2 nm and 1–4 nm, respectively. Thus, the interstitial and internal nanopores were named as narrow and wide nanopores, respectively. The X-ray photoelectron spectroscopies in Fig. 1c and transmission electron microscopic images in Fig. 1d indicated that geometrical and chemical structures were rarely changed by the partial oxidation of as-grown CNHs. The O/C ratios were approximately 5% and those CNHs had less surface oxygen groups. Thus, we discussed the change of selectivity by nanopore size. Figure 1e,f shows N2 and SF6 adsorption isotherms measured for the CNHs at 273 K. The N2 adsorption isotherms measured at 273 K were Henry-type, and the adsorbed amounts were small. In contrast, SF6 was adsorbed well in these nanopores even at 1 atm, because of the strong intermolecular interactions of the SF6, corresponding to P/P0 = 0.07. Because the amounts of SF6 adsorbed in the interstitial and internal nanopores were significantly different, the SF6 was less densely packed in the interstitial, narrow nanopores with a diameter of 0.7 nm.

Bottom Line:
The high selectivity of SF6 over N2 was observed only in the low-pressure regime in the interstitial 0.7 nm nanopores; the selectively was significantly decreased at higher pressures.In contrast, the high selectivity was maintained over the entire pressure range in the internal 2.9-nm nanopores.These results showed that the wide carbon nanopores were efficient for the separation of SF6 from the mixed gas.

ABSTRACTSF6 and SF6-N2 mixed gases are used widely as insulators, but such gases have high greenhouse gas potential. The separation of SF6 from SF6-N2 mixed gases is an inevitable result of their use. Single-walled carbon nanohorns (CNHs) were used here for a fundamental study of the separation of SF6 and N2. The diameters of the interstitial and internal nanopores of the CNHs were 0.7 and 2.9 nm, respectively. The high selectivity of SF6 over N2 was observed only in the low-pressure regime in the interstitial 0.7 nm nanopores; the selectively was significantly decreased at higher pressures. In contrast, the high selectivity was maintained over the entire pressure range in the internal 2.9-nm nanopores. These results showed that the wide carbon nanopores were efficient for the separation of SF6 from the mixed gas.